When boring or facing box workpieces, such as the box body of a reduction box, the end faces of the box workpieces are required to be set apart, therefore, the box workpieces can be clamped using only the sides of the box workpieces. However, the existing tools for clamping act on the sides of workpieces through pressing blocks, the pressing blocks are often difficult to tightly contact with the sides of the box workpieces as the sides of the box workpieces usually have a draft caused by that the box workpieces are generally processed in a die-casting way. As a result, the box workpieces cannot be tightly pressed, which leads to a location moving possibility of the box workpieces during a processing procedure, thus affecting the processing quality of the box workpieces.